1. Field of the Invention
The present invention relates generally to a gas sensor which uses a solid electrolyte for determining the concentration of a component in a gaseous fluid, and more particularly to such a gas sensor which is capable of stably providing a sharp characteristic curve and thereby effecting the measurement with significantly improved accuracy.
2. Disclosure of the Prior Art
There has been known a device which incorporates an electrochemical cell using a solid electrolyte. For example, such an electrochemical device is used as an oxygen sensor having an electrochemical cell which consists of an oxygen-ion conductive solid electrolyte such as zirconia ceramics, and a pair of porous electrodes, for determining the concentration of oxygen in an exhaust gas produced by an internal combustion engine of an automotive vehicle. In this type of sensor, an electrochemical pumping action is performed based on the reaction of the electrodes which occurs while an electric current is applied between the pair of electrodes. In the meantime, one of the porous electrodes is held in communication with a measurement gas in an external measurement-gas space, via suitable diffusion-resistance means such as a pin hole, a thin flat space or a porous ceramic layer, which provides a predetermined resistance to a diffusion of the measurement gas. The sensor provides an output in the form of a pumping current which corresponds to the oxygen concentration of the external measurement gas. Also known are electrochemical devices or gas sensors or detectors adapted to detect hydrogen, carbon dioxide, fuel gases, etc., by utilizing the principle based on the electrochemical pumping action and the diffusion resistance, as practiced in the oxygen sensor discussed above.
In one type of gas sensor using such an electrochemical cell (pumping cell) capable of performing an electrochemical pumping operation, the solid electrolyte body constituting the electrochemical cell is made porous so that it may function as a porous ceramic layer or diffusion-resistance means having a predetermined diffusion resistance. In this case, the porous solid electrolyte body is formed as a comparatively bulky mass on which a pair of electrodes are integrally formed. Accordingly, the measurement gas which diffuses through the interior of the bulky solid electrolyte mass to one of the electrodes takes different diffusion paths, whereby there arises a gradient in the concentration of the diffused measurement gas on the electrode. This gradient indicates an undesirable polarization characteristic, that is, insufficient sharpness of a characteristic curve (pumping current-pumping voltage curve) obtained by a pumping operation of the pumping cell, which results in a problem of inaccurate measurement of the measurement gas by the gas sensor.
Further, since the measurement gas diffuses also through the porous first and second electrodes of the electrochemical cell, the overall diffusion resistance of the cell is influenced by the gas permeability of the electrodes which may vary from time to time. This is another problem experienced in the known gas sensor discussed above.